The invention relates to a developing unit including a developing electrode and in which a toner is brought into contact with the surface of a photosensitive member carrying an electrostatic latent image to provide developing thereof, and more particularly to a self-biased developing unit for electrophotography which automatically controls a bias potential to be applied to the developing electrode and which is of a suitable magnitude to provide a copy free from background smearing.
It is known that a developing technique; such as a cascade process which employs no developing electrode or involved application of a bias; unfavorably produces an edge effect on a copy having a high proportion of image areas such as photographs, even though it may be successfully employed for a copy having a reduced proportion of image areas, such as letterprints. A magnetic brush developing technique which employs a dry developer, or a developing technique which employs a developing electrode together with a wet developer, can be successfully used to provide a satisfactory copy from both originals having a high and a low proportion of image areas, by applying a bias potential to the magnetic brush or the developing electrode which is, by a fixed amount, higher than the background potential of the surface of the photosensitive member. However, this only applies when the exposure is properly chosen, and, if the exposure is improper, there cannot be obtained a satisfactory copy. Since a document being copied frequently has a varying background density or a varying color, it is then necessary to choose a proper exposure for each individual original to be copied in order to provide a copy which is free from background smearing, by using the developing process which employs the above-mentioned fixed bias. The determination of the exposure is made only after a copy is actually produced, so that there may result a waste of unsuccessful copies before a satisfactory copy can be obtained.
To overcome the above disadvantage, there has been proposed a self-biased developing technique, commonly referred to as self auto-bias technique. With this technique, an electric charge on the surface of the photosensitive member, which varies with an original to be copied, is detected to automatically control a suitable bias potential applied to the developing electrode in accordance with the detected magnitude of the charge. The auto-bias technique can be implemented in a relatively complex arrangement which requires an external source and an electrical arithmetic circuit to control and apply an optimum bias potential, thus supplying a forced bias. Though positive in action, it is complex in arrangement and is also expensive. A simple arrangement according to the auto-bias technique is also known in an arrangement which uses a developing electrode of a floating potential type. As shown in FIG. 1, which represents an electrical equivalent circuit, an average surface potential Vs of a latent image on the photosensitive member is divided by a voltage divider comprising a resistance R1, of a developer present between the photosensitive member and the developing electrode, and a resistance R2, existing between the developing electrode and the body of the machine, thereby inducing a potential V.sub.D which is applied to the developing electrode as a bias potential. The relationship between the average surface potential Vs of the latent image and the potential V.sub.D of the developing electrode is depicted in FIG. 2, and can be represented as follows: ##EQU1## By a suitable choice of R1 and R2, there can be produced automatically an adequate potential, for the developing electrode, which depends on the varying potential of the latent image. However, while the described technique provides a satisfactory copy for an original having a high proportion of image areas, it produces a copy with background smearing from a document having a lower proportion of image areas. In addition, an original of a reduced brightness, such as photographs, will result in a generally whitish copy. Finally, degradation in the toner quality by aging or environmental change may cause a change in the resistance R1 presented by the developer, thus making it difficult to maintain a satisfactory copy quality over a prolonged period of use.